National university corporation
UNIVERSITY OF YAMANASHI

Back ground and objective
The objective of this study are to research gas mixing process
and to develop the prevention technology of air ingress.

STUDY ON AIR INGRESS PROCESSES DURING A
DEPRESSURIZATION ACCIDENT OF VHTR

It is necessary to prevent air Ingress or oxidation of graphite
at pipe rupture accident of Very High Temperature Reactor.

Contents
1. Introduction
2. Experimental apparatus and numerical model
3. Experimental and numerical results and discussion
4. Conclusions

Even if the pipe rupture accident occurs, ingress of air can
prevent by injecting helium gas.

Air ingress scenario in the case of the horizontal pipe break
1. Pipe rupture at connecting pipe between RPV and gas
turbine.
2. Helium gas blows off from the RPV.
3. Pressure in the reactor equalized to the one in thecontainment or confinement vessel.
4. Buoyancy force produce by the temperature difference
between inside and outside passage in the RPV.
5. Natural circulation of air will produce. (depend on
temperature profile or geometrical condition)
6. Graphite of reactor component will react with ingress air.

Tetsuaki TAKEDA
Graduate School of Engineering. Dept. of Mechanical Engineering
University of Yamanashi
International Conference on High Temperature Reactor Technology HTR 2018,
October 8-10, 2018, Warsaw, Poland
International Conference on High Temperature Reactor Technology, HTR 2018, Oct. 8-10, 2018, Warsaw, Poland, HTR2018-180

Schematic diagram of GTHTR300C
Designed by JAEAInternational Conference on High Temperature Reactor Technology, HTR 2018, Oct. 8-10, 2018, Warsaw, Poland, HTR2018-180

2

Schematic drawing of the HTTR and model of coolant passages

Back ground• A hot leg consists of an inner passage of a coaxial duct, a high-temperature outlet duct, a hightemperature plenum and fuel cooling channels.

It is necessary to prevent air Ingress oroxidation of graphite at pipe rupture
accident of High Temperature Reactor• A cold leg consists of an annular passage of the coaxial duct, a bottom cover and an annular passage
between the reactor pressure vessel and permanent reflector.
• As the hot and cold legs are connected at the top space, they make a kind of reverse U-shaped tube.

Air Ingress Scenario in the case of thehorizontal pipe break
1. Pipe rupture at connecting pipe between
RPV and gas turbine.
2. Helium gas blows off from RPV.
3. Pressure in the reactor equalized to the one
in the containment or confinement vessel.
4. Buoyancy force produce by the temperature
difference between inside and outside
passage in the RPV.
5. Natural circulation of air will produce.
(depend on temperature profile orgeometrical condition)
6. Graphite of reactor component will reactwith ingress air.

Schematic diagram of GTHTR300C
Designed by JAEAInternational Conference on High Temperature Reactor Technology, HTR 2018, Oct. 8-10, 2018, Warsaw, Poland, HTR2018-180

3International Conference on High Temperature Reactor Technology, HTR 2018, Oct. 8-10, 2018, Warsaw, Poland, HTR2018-180

4

Schematic drawing of the GTHTR300C and model of coolant passages

Influence that localized natural convection in the vertical channels withdifferent temperature exerts on onset time of natural circulation
The experiment has been carried out toresearch mixing process of two component
gases and onset time of natural circulation of air.
Light gas

When one side wall is heated and the other sidewall is cooled in a vertical slot, a localized
natural convection is generated.
Heavy gas will diffuse into the both vertical slots
at the same time, and then time elapsed,
natural circulation through the passage will be
generated finally.

Heavy gas
Molecular diffusion

Purpose of this study
To investigate an onset time of natural
circulation and a mixing process of two
component gases by using 3D numericalanalysis.
International Conference on High Temperature Reactor Technology, HTR 2018, Oct. 8-10, 2018, Warsaw, Poland, HTR2018-180

5The flow regime of this localized
natural convection is ranging fromconduction regime to boundary
layer regime.
6

International Conference on High Temperature Reactor Technology, HTR 2018, Oct. 8-10, 2018, Warsaw, Poland, HTR2018-180

Experimental procedure

Experimental apparatus
 The apparatus consists of two vertical slots,
connecting passage and storage tank.

1. Heavy gas is filled with apparatus.

 Vertical slots and storage tank wereseparated by partition plate.

2. Partition plate close.
3. Light gas inject from top of theapparatus.

 The left side vertical slot consists of a heatedwall and a cooled wall.

4. Two vertical wall of the left handside slot is heated and cooled.

 The right side vertical slot consists of twocooled walls.

5. When the steady state wasestablished, the partition plate
open.

Light gas

6. Experiment starts.
Heavy gasInternational Conference on High Temperature Reactor Technology, HTR 2018, Oct. 8-10, 2018, Warsaw, Poland, HTR2018-180

7International Conference on High Temperature Reactor Technology, HTR 2018, Oct. 8-10, 2018, Warsaw, Poland, HTR2018-180

8Numerical model

Numerical model
PHOENICS: three-dimensional CFD code

PHOENICS：three‐dimensional CFD codeAnalytical domain
X : 548, Y : 398, Z : 846 mm

Analytical domain
X : 548, Y : 398, Z : 846 mm

Analytical gridx: 122, y: 20, z: 118 (total
cells : 266680)

Analytical gridx: 122, y: 20, z: 118 (total cells : 266680)
Boundary condition

Boundary condition
The outside of the heatedwall and the cooled wallassumed an adiabatic wall.
The other walls assumed
naturalconvection heattransfer boundary condition.

The outside of the heated wall and thecooled wall assumed an adiabatic wall.
The other walls assumed naturalconvectionheattransferboundary
condition.

Z
X

Others
Standard density : ρ0
 Buoyancy : (ρ-ρ0)gVX

Others
Standard density : ρ0
→Buoyancy : (ρ-ρ0)gV
9International Conference on High Temperature Reactor Technology, HTR 2018, Oct. 8-10, 2018, Warsaw, Poland, HTR2018-180

Numerical method

Initial condition of steady state:
Heavier and lighter gases were filled.
The vertical walls of the left side slot washeated and cooled.
Initial condition of unsteady state:
The result obtained by steady statecalculation.

Density [kg/m 3] (20ºC, 1atm)

International Conference on High Temperature Reactor Technology, HTR 2018, Oct. 8-10, 2018, Warsaw, Poland, HTR2018-180

Two component gases(light-heavy)

Diffusion coefficient[cm2/s]
0.20

Gases

Density [kg/m 3] (20ºC, 1atm)

Helium (He)0.164

Neon (Ne)

0.838

Nitrogen (N2)

1.17

Argon (Ar)1.64

Two component gases(light-heavy)

Diffusion coefficient[cm2/s]

N2-Ar

0.20

Ne-Ar

0.32

He-N2

0.68

He-Ar

0.73

Helium (He)

0.164

N2-Ar

Neon (Ne)

0.838

Ne-Ar

0.32

Temperature difference between wall [K]Nitrogen (N2)

1.17

He-N2

0.68

30，50，70，100

Argon (Ar)

1.64

He-Ar

0.73

International Conference on High Temperature Reactor Technology, HTR 2018, Oct. 8-10, 2018, Warsaw, Poland, HTR2018-180

10Two component gases and temperature differenceNumerical method:
1. Steady state calculation:
Natural convection was generated bytemperature difference between thevertical walls.
2. Unsteady state calculation:
Partition plate was opened at 0 sec.
Calculation step ：0.01sec/step・・・ 0~10sec
0.05sec/step ・・・ 10sec~

Gases

Z

11

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12

Numerical result (change of gas temperature)

Numerical result (change of gas temperature)

Z[mm]

(1)

(10)

(3)

200

(2)
(4)
(5)

(11)

(7)
(8)

200

(6)

(9)
75

Z

90 50 50 50 50 50 50 50 50

Z[mm]

Z[mm]

(12)

(He-Ar, ΔT=100K)

Z[mm](He-Ar, ΔT=100K)
13International Conference on High Temperature Reactor Technology, HTR 2018, Oct. 8-10, 2018, Warsaw, Poland, HTR2018-180

Comparison between experiment and numerical analysis

Numerical result (change of velocity)
Z[mm]

14

International Conference on High Temperature Reactor Technology, HTR 2018, Oct. 8-10, 2018, Warsaw, Poland, HTR2018-180

experimentX

numerical analysis
Z[mm]

Z[mm]

15℃

5℃
Z[mm]

(He-Ar, ΔT=100K)
International Conference on High Temperature Reactor Technology, HTR 2018, Oct. 8-10, 2018, Warsaw, Poland, HTR2018-180

Z[mm](He-Ar, ΔT=100K)
15International Conference on High Temperature Reactor Technology, HTR 2018, Oct. 8-10, 2018, Warsaw, Poland, HTR2018-180

16

Numerical result (distribution of mole fraction)

Numerical result (distribution of gas velocity)(He-Ar, ΔT=100K)

Velocity [m/s](a)30min(b)52min
Localized natural convection Just before naturalgenerated in the left side circulation generated.
slot.

(d)54minNatural circulation generated
through the passage.

17

International Conference on High Temperature Reactor Technology, HTR 2018, Oct. 8-10, 2018, Warsaw, Poland, HTR2018-180(He-Ar, ΔT=100K)

Mole fraction(Ar)(a)30min
Mixing by molecular
diffusion was promotedby the localized naturalconvection in the left sideslot.

(b)52minJust before natural
circulation generated.

(d)54minNatural circulation generated
through the passage andmolefraction
becameuniform.
18

International Conference on High Temperature Reactor Technology, HTR 2018, Oct. 8-10, 2018, Warsaw, Poland, HTR2018-180

Comparison between experiment and numerical analysis
Onset time of natural circulation

Relationship between onset time of natural circulation and Gr number

Experiment
ΔT [K] He/Ar [min]
30
90
50
75
70
60
100
① 55

N₂/Ar [min]
180
140
90
③ 70

Generation time （analysis）
ΔT [K]
He/Ar [min]
30
84
50
76
70
65
100
57

He/N₂ [min]
103
80
77
63

Ne/Ar [min]
128
115
109
93

N₂/Ar [min]
164
130
97
76

N₂/Ar [min]
164
130
97
③ 76

Gr number （in the left slot）
ΔT [K]
He/Ar (×10⁴)30
0.045 ～ 0.95
50
0.068 ～ 1.4
70
0.079 ～ 1.7
100
0.10 ～ 2.2
Density ratio
1/10

He/N₂ (×10⁴)
0.055 ～ 0.84
0.076 ～ 1.3
0.090 ～ 1.5
0.11 ～ 1.8
1.4/10

Ne/Ar (×10⁴)
0.44 ～ 2.1
0.75 ～ 2.7
0.84 ～ 2.9
1.2 ～ 4.4
7/10

N₂/Ar (×10⁴)
2.7 ～ 3.2
4.1 ～ 5.0
4.8 ～ 6.0
6.8 ～ 11
5/10

Numerical analysis
ΔT [K] He/Ar [min]
30
84
50
76
70
65
100
① 52

He/N₂ [min]
105
76
72
② 60

He/N₂ [min]
103
80
77
② 63

Ne/Ar [min]
140
130
100
④ 85

Ne/Ar [min]
128
115
109
④ 93

 Onset time of natural circulation became short with increasing
temperature difference.
 Gr number increases with increasing temperature difference.

Difference between experiment and numerical analysis
ΔT [K] He/Ar [%]He/N₂[%]
Ne/Ar [%]
N₂/Ar [%]
30
-6.67
-2.22
-8.33
-8.98
50
1.78
5.26
-11.5
-6.90
70
8.33
6.48
8.50
7.59
100
3.64
4.44
9.41
8.57

Onset time of natural circulation became short because Gr number increased.
Natural convection became strong and mixing of gases was promoted.

－：faster than experimentInternational Conference on High Temperature Reactor Technology, HTR 2018, Oct. 8-10, 2018, Warsaw, Poland, HTR2018-180

19International Conference on High Temperature Reactor Technology, HTR 2018, Oct. 8-10, 2018, Warsaw, Poland, HTR2018-180

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Relationship between onset time and diffusion coefficient

Relationship between onset time and diffusion coefficient

Generation time （analysis）
ΔT [K]
30
50
70
100

He/Ar [min]
① 84
① 76
① 65
① 57

Generation time （analysis）
He/N₂ [min]
② 103
② 80
② 77
② 63

Ne/Ar [min]
③ 128
③ 115
④ 109
④ 93

N₂/Ar [min]
④ 164
④ 130
③ 97
③ 76

ΔT [K]
30
50
70
100

He/Ar [min]
① 84
① 76
① 65
① 57

He/N₂ [cm/s²]
0.678
0.712
0.763
0.824

Ne/Ar [cm/s²]
0.325
0.326
0.355
0.374

N₂/Ar [cm/s²]
0.205
0.211
0.228
0.234

ΔT [K]
30
50
70
100

He/Ar
Ne/Ar [cm/s²](×10⁴)
0.742
0.44
～ 2.1
0.779
0.75
～ 2.7
0.833
0.84
～ 2.9
1.20.888～ 4.4

Gr number

Diffusion coefficient
ΔT [K]
30
50
70
100

He/Ar [cm/s²]
0.742
0.779
0.833
0.888

He/N₂ [min]
② 103
② 80
② 77
② 63

 When temperature difference was 30 and 50 K, onset time became
short with increasing diffusion coefficient.
 Onset time of N₂/Ar and that of Ne/Ar were reversed at 70 and 100 K.

N₂/Ar [min]
④ 164
④ 130
③ 97
③ 76

Diffusion coefficient
Ne/Ar [cm/s²]
0.325
0.326
0.355
0.374

N₂/Ar [cm/s²]
0.205
0.211
0.228
0.234

 When temperature difference was 30 and 50 K, onset time became
short with increasing diffusion coefficient.
 Onset time of N₂/Ar and that of Ne/Ar were reversed at 70 and 100 K.
This is because Gr number of N₂/Ar was larger than that of Ne/Ar, so naturalconvection became strong.

21

International Conference on High Temperature Reactor Technology, HTR 2018, Oct. 8-10, 2018, Warsaw, Poland, HTR2018-180

He/N₂[cm/s²]
N₂/Ar (×10⁴)
2.70.678～ 3.2
4.10.712～ 5.0
4.80.763
～ 6.0
0.824
6.8
～ 11

Ne/Ar [min]
③ 128
③ 115
④ 109
④ 93

Influence that combination of two component mixed gas exerts on onset
time of natural circulation

When temperature difference is small, onset time depended mainly on
diffusion coefficient.
When temperature difference is large , onset time depended not only
on diffusion coefficient but also on localized natural convection.
International Conference on High Temperature Reactor Technology, HTR 2018, Oct. 8-10, 2018, Warsaw, Poland, HTR2018-180

22

3 parallel vertical channels having different temperature

Figure shows the onset time of natural circulation against of the wall temperatureof the high-temperature side passage.
Diffusion coefficient increase360

Onset time of natural circulation [min]Nitrogen/Argon 7/10
300

Neon/Argon 5/10
Helium/Nitrogen 1.5/10

240Helium/Argon 1/10
180

120

60

Gr number increase
0
0

50
100
Heated side wall temperature [C]International Conference on High Temperature Reactor Technology, HTR 2018, Oct. 8-10, 2018, Warsaw, Poland, HTR2018-180

150

23

International Conference on High Temperature Reactor Technology, HTR 2018, Oct. 8-10, 2018, Warsaw, Poland, HTR2018-180

24

Influence that temperature difference of 3 parallel vertical channels exerts
on onset time of natural circulation

Influence that graphite oxidation on the reverse U-shaped vertical channel
exerts on onset time of natural circulation

360

240
3ch. same temperature

162-162-162

Helium/Air with oxidation
Onset time of natural circulation [min]Onset time of natural circulation [min]2ch. same and 1ch. low temperature
2ch. same and 1ch. high temperature300

3ch. different temperature

200-199-209

164-115-121

240

243-242-243
205-130-135

164-162-108

204-202-132

281-279-280

205-133-131
243-202-169

164-162-85

284-283-283
245-145-142

244-241-170

282-240-209

202-200-167

180

281-280-211

284-181-213
244-167-167

246-319-245

285-174-174
245-241-118

323-321-322

120

Onset time of natural circulation
was not so much affected withgraphite oxidation.

322-281-248

283-207-216
323-243-210
284-281-133

Helium/Nitrogen without oxidation
180319-317-250

60

324-211-209

300

120
100

150

200

250

300

400

350

500

600
700
Average temperature [C]

800

900

Average temperature of the pipe wall [C]

25International Conference on High Temperature Reactor Technology, HTR 2018, Oct. 8-10, 2018, Warsaw, Poland, HTR2018-180

Onset time of natural circulation against wall temperature of the high-temperature sideof the reverse U-shaped passages (height of heated part is less than 1m)

International Conference on High Temperature Reactor Technology, HTR 2018, Oct. 8-10, 2018, Warsaw, Poland, HTR2018-180

26

Experimental apparatus simulated HTTR

Figure shows the onset time of natural circulation obtained by 3 apparatus. Three apparatus are
the reverse U-shaped tube, three parallel channels, and vertical parallel walls. The height of theheated part is less than 1m.
When natural circulation
was generated, the onset
time of natural circulation
becomes short.

1000
Helium/Air with oxidation

Onset time of natural circulation [min]Helium/Nitrogen without oxidation
Nitrogen/Argon 7/10

Temperature difference

Neon/Argon 5/10

100Helium/Nitrogen 1.5/10
Helium/Argon 1/10
3ch. same temperature
2ch. same and 1ch. low temperature
2ch. same and 1ch. high temperature

10

3ch. different temperature
Helium/Nitrogen 1.5/10
Helium/Air 1.3/10
Nitrogen/Argon 7/10

The onset time of natural
circulation becomes longwhen the phenomenon
is governed mainly bymolecular diffusion.

When not only the localized
natural convection but also
natural circulation was generated,the onset time of natural
circulation becomes short.

Length of vertical passage
1
1

10
100
Average temperature of the channel wall [C]International Conference on High Temperature Reactor Technology, HTR 2018, Oct. 8-10, 2018, Warsaw, Poland, HTR2018-180

1000
27

International Conference on High Temperature Reactor Technology, HTR 2018, Oct. 8-10, 2018, Warsaw, Poland, HTR2018-180

28

Influence that temperature difference in the vertical graphite passage exerts on
onset time of natural circulation

Influence that vertical length of the passage under the core exerts on
onset time of natural circulation
100000288

Helium/Air with oxidation

Helium/Air with oxidation

Different temperature

Different temperature 71.4
Different temperature 74.6

Onset time of natural circulation [min]Onset time of natural circulation [h]240Different temperature 81.3
Different temperature 80.6
192

Different temperature 84.7

144

When natural circulation was generated, theonset time of natural circulation becomes short.

96

The onset time of natural circulation becomeslong when the phenomenon is governed mainly
by molecular diffusion.

Helium/Air with oxidation

10000

Helium/Nitrogen without oxidation
Nitrogen/Argon 7/10
Neon/Argon 5/10

1000

Helium/Nitrogen 1.5/10
Helium/Argon 1/10
3ch. same temperature100

2ch. same and 1ch. low temperature
2ch. same and 1ch. high temperature
3ch. different temperature

10

Helium/Nitrogen 1.5/10
Helium/Air 1.3/10
Nitrogen/Argon 7/10

48

1
300

400

500
600
700
800
Average temperature of channel wall [C]

900

1000

International Conference on High Temperature Reactor Technology, HTR 2018, Oct. 8-10, 2018, Warsaw, Poland, HTR2018-180

1

29

10
100
Average temperature of the channel wall [C]

1000

30

International Conference on High Temperature Reactor Technology, HTR 2018, Oct. 8-10, 2018, Warsaw, Poland, HTR2018-180

Experimental procedure (He injecting)

Proposed system in the case of GTHTR300 (JAEA’s idea)

In order to investigate of preventing natural circulation flow by injecting helium
gas, an experiment has been done as follows.

Heated wallHe

1. Heavy gas is filled with apparatus.
2. Two vertical wall of the left handside slot is heated and cooled.
3. Natural circulation flow are
produced. When the steady stateis established, helium gas injects
from the top of the apparatus.
4. Experiment starts.
5. Natural circulation will be stopped.
6. After the time elapsed, natural
circulation will be reproduced
suddenly.

Helium gas

Cooled wall
Air
Ref. Yan, X. L. et al. (2008). "A study of air ingress and its prevention in HTGR, " Nucl.
Technology, 163, No.3, pp.401‐415.
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31International Conference on High Temperature Reactor Technology, HTR 2018, Oct. 8-10, 2018, Warsaw, Poland, HTR2018-180

32

Experimental results regarding re-onset of NC

Stop time of natural circulation [s]

45
40
35

20
15
10
5
0
2

decrease

25⊿30K

Temperature difference

30

⊿70K

increase

Onset time of natural circulation [min]Re-onset time of natural circulation increased withincreasing the injection volume of He

⊿50K

⊿100K
In order to prevent natural circulation flow,
the amount of injecting He increased withincreasing temperature difference
3

4
5
6
7
8
9
Helium gas injection volume [%]

10

9000
7200

3600
1800

Helium gas

0

11

2

33

International Conference on High Temperature Reactor Technology, HTR 2018, Oct. 8-10, 2018, Warsaw, Poland, HTR2018-180

40K
60K
80K

5400

Temperature difference

Relationship between elapsed time and injection volume

Air Ingress Scenario in the case of the horizontal pipe break

4

6
8
10
Volume rate [%]

12

14

34

International Conference on High Temperature Reactor Technology, HTR 2018, Oct. 8-10, 2018, Warsaw, Poland, HTR2018-180

Experimental apparatus simulating GTHTR300 series

HeaterHe

He

Diffusion

Air

Air

Air
International Conference on High Temperature Reactor Technology, HTR 2018, Oct. 8-10, 2018, Warsaw, Poland, HTR2018-180

h₂

h₁
D₂

V₁
V₂

d₂

v₂

Experimental apparatus• If the localized naturalconvection occur inside thechannel, it is difficult toestimate not only the densitychange of gas mixture but
also the onset time of natural
circulation through the reactor.

NaturalConvection

d₁

D₁• Thus, air will transport to thecore by mainly molecular
diffusion.

He

Vesselv₁• As the buoyancy force will besmall, the natural circulation
flow will not produce under
the condition of this density
distribution.

D₃
D₄• Buoyancy force will produce between the hot and cold legs.

H₁
H₂• The density stratified fluid layer will be formed.

d₃d₄• After the pipe ruptures, air will flow into the bottom part of the RPV by the counter-current
flow.

• After the time elapses, thenatural circulation may occur
suddenly.

35

Sectional view of
GTHTR300

Scaling of GTHTR300 and experimental apparatus

GTHTR300/Apparatus

Aspect ratio of pressure vessel Height/Outer diameter [H1/D1]
Aspect ratio of outer cylinder Height/Outer diameter [h1/d1]

1.86 / 1.53

Aspect ratio of core Height/Outer diameter [H2/D2]
Aspect ratio of Inner cylinder Height/Outer diameter [h2/d2]

1.50 / 1.43

Diameter ratio of primary double coaxial pipe [D3/D4]
Diameter ratio of horizontal double coaxial pipe [d3/d4]

1.30 / 1.50

Volume ratio of pressure vessel & core [V1/V2]
Volume ratio of outer cylinder & inner cylinder [v1/v2]

5.00 / 4.60

International Conference on High Temperature Reactor Technology, HTR 2018, Oct. 8-10, 2018, Warsaw, Poland, HTR2018-180

36

Experimental apparatus

Results of temperature profiles in the apparatus
Heat input: 324 [W] (10995 [W/m²])
Temperature fluctuatedfrom 2 to 10 K

Approach tosteady state

(36)
(35)

(32)

(31)

Temperature
difference is
15 to 55 K

100 mm

350mm
283.4mm
190mm
139.8mm(38)

Cooling water inlet

Heat insulation

(31)
(32)
(35)
(36)
(38)

Outer cylinder(Water cooling jacket entered)Outer pipe of horizontal double pipe

Inner cylinder
410mm

Cooling water outlet

255mm

Inner pipe of horizontal double pipe

Cartridge heaterzTemperature fluctuatedfrom 1 to 2 K

Valve opened

420 mm100mm

Flow meterx

Ball valve
Gas sampling and supply port

(31)
(36)
(32)

Localized natural convection will be
generated at the top of inner cylinder

Thermocouple

(38)

y

37

International Conference on High Temperature Reactor Technology, HTR 2018, Oct. 8-10, 2018, Warsaw, Poland, HTR2018-180

(35)

x

Results of inlet velocity change at the horizontal pipe

Results of molar fraction change in the apparatus

Heat input: 324 [W] (10995 [W/m²])

Heat input: 324 [W] (10995 [W/m²])

He

Counter-current flow
Air ingress

Natural circulation of air
Blow-down of helium & airCounter-current flow

(1)
(4)
(8)
(11)
(15)
(16)

(8)

(11)

He

(1)

Natural circulation of air
Valve opened

Air

Blow-down of helium & air

Hez

Dotted line shows an estimated curve because a detectable
range of molar fraction of helium is lower than 50%.

He

(15)

(16)

Air

Blow-down of helium gas by a counter-current flow.

Natural circulation of air

Air

z
x

(1)

Blow-down of helium & airCounter-current flow

(4)

(1)
(4)
(8)
(11)
(15)
(16)

Dotted line shows an estimated curve because a detectable
range of molar fraction of helium is lower than 50%.

Blow-down of helium and air. (mixture gas)Airx

Air ingress
Valve opened

Blow-down of helium gas that exists under the
broken part of the pipe.

38

International Conference on High Temperature Reactor Technology, HTR 2018, Oct. 8-10, 2018, Warsaw, Poland, HTR2018-180

Blow-down of helium gas by a countercurrent flow.
Blow-down of helium gas that exists underthe broken part of the pipe.
Blow-down of helium and air. (mixture gas)
Onset of natural circulation of air.

Onset of natural circulation of air.
International Conference on High Temperature Reactor Technology, HTR 2018, Oct. 8-10, 2018, Warsaw, Poland, HTR2018-180

Valve opened
39

International Conference on High Temperature Reactor Technology, HTR 2018, Oct. 8-10, 2018, Warsaw, Poland, HTR2018-180

40

Conclusion The onset time of the natural circulation depended more on molecular diffusion
than the strength of localized natural convection when the temperature difference
was small.
 On the other hand, the onset time of natural circulation depended not only on
molecular diffusion but also on localized natural convection when the temperature
difference between two vertical walls was large.
 These flow characteristics will be the same as those of phenomena generated in
the passage between a permanent reflector and a pressure vessel wall of the
GTHTR-300C.
 In order to prevent a large amount of air ingress into the reactor by injecting helium
gas, we are planning to analyze the method for preventing air ingress by helium
canister during a depressurization accident in the GTHTR-300C system.
 In addition, we are now doing the experiment by the double coaxial cylindricalapparatus and also are planning to carry out 3-D numerical analysis of air ingress
when the horizontal primary pipe ruptured.
International Conference on High Temperature Reactor Technology, HTR 2018, Oct. 8-10, 2018, Warsaw, Poland, HTR2018-18041